Diagnostics and testing of rotating electrical machines - DNV Kema

DNV KEMA sERVING THE ENERGy INDusTRy 

Non-destructive testing & plant diagnostics 

Diagnostics and testing of 

rotating electrical machines 

PREPARE 

AND 

PREVENT

02 

ENERGY 


PREPARE AND PREVENT 

Diagnostics and testing of rotating electrical machines 

Rotating machines are of vital importance to many business processes. As the demand for energy continues to grow, 

operational continuity is increasingly important. The failure of rotating machinery used in energy production, waste 

processing and other industrial processes not only has financial and operational consequences for the owners, but in 

many cases also has implications for the community at large. DNV KEMA Energy & Sustainability possesses enormous 

specialist expertise, which is made available to help clients keep their vital processes running. 

Responsible use of aging machines 

Being vital to many industrial processes, rotating machines are 

designed to operate reliably for years on end. However, as time 

goes by, attention inevitably is focused on the condition of aging 

machines and how much longer they can be expected to go on 

providing problem-free service. With a wide range of diagnostic 

techniques at its disposal, DNV KEMA can help you minimize 

the operating and financial cost of finding answers. Our advice 

will enable you to identify the most cost-effective and responsible 

maintenance strategy and operational approach, now and in the 

future. 

The need to manage costs makes it sensible to consider whether 

rotating machines can remain in service longer without undue 

outage risk or excessive expenditure on maintenance. 

After all, the serviceability of a component depends not only on 

its age, but also on the way it is operated and maintained, and 

what has happened to it in the past. 

DNV KEMA can provide answers to the questions that you are 

liable to face, such as: 

■■ 

What condition is our machinery actually in 

■■ 

Can we make even better use of our rotating machinery 

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How much longer can our machinery remain in service 

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Can we reasonably postpone capital expenditure 

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Is it worthwhile repairing our machinery, or should we replace it 

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How big a risk would we be taking if we took no action yet 

■■ 

What was the cause of the damage our machine has 

unexpectedly suffered, and who is responsible


ENERGY 03 

It is not always easy to give answers to such questions. A rotating 

machine is, after all, merely part of a large and complex process. 

It is expected to simply go on doing its job, and analyzing its 

condition is not part of your core activities. Although most 

companies that operate rotating machinery are reasonably 

knowledgeable about the working of their assets, precise 

condition determination and residual service life assessment are 

the work of specialists. Specialists for whom the testing and 

inspection of machinery are everyday tasks and who have an 

armory of diagnostic techniques at their disposal. Specialists 

who bring many years of experience to the diagnosis of your 

installation and who are able to consider your machinery from 

an impartial perspective. 

Specialists that DNV KEMA can deploy on your behalf. At DNV 

KEMA, we have the knowledge, the methodological expertise 

and tools to provide you with the backup and support that you 

need to maximize operational efficiency and reliability. Now and 

in the future. 

Design 

DNV KEMA can help you to obtain exactly the right machine for 

your company or process. A supplier is responsible for delivering 

products and services that meet your specified requirements. 

It is therefore important that you specify your requirements 

accurately. Well-formulated DNV KEMA (S-)specifications 

provide a basis for the assessment of a supplier’s design. Good 

specifications are the starting point for a sound design, which 

is essential for obtaining a machine that performs optimally 

throughout its service life. 

Manufacture 

During manufacture, a number of topics are of particular 

importance: the selection of materials and the actual process 

of manufacture and the mechanisms with which you, as the 

(prospective) owner, can obtain assurance regarding the 

quality of the finished machine. DNV KEMA can support you 

during manufacture by providing ongoing quality control, by 

witnessing the acceptance tests at the factory and on site, and by 

supervising the commissioning.

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ENERGY 


■■ 

Quality control: checking the manufacturing process by 

inspection and testing. Various components are tested during 

manufacture. The manufacturer’s progress, quality assurance 

measures and results are also assessed. 

■■ 

Tests (FAT and SAT): DNV KEMA performs its own tests or 

witnesses testing by others to check conformance to the design 

specifications. During factory acceptance testing (FAT), type 

tests and routine tests are performed to verify that the rotating 

machine has been built in accordance with the specifications. 

During site acceptance testing (SAT), the quality of the 

delivered installation is checked to ascertain whether it fullfils 

the applicable requirements. 

■■ 

Commissioning: once all the separate components have been 

installed and tested, DNV KEMA will witness and assess testing 

of the complete installation under various normal and/or 

extreme operating scenarios, defined by consultation. 

From design to decommissioning 

The life cycle of any machine may be divided into the following phases: 

■■ 

Design 

■■ 

Manufacture 

■■ 

Operation 

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Conservation 

■■ 

Decommissioning 

Operations 

Our approach to condition assessment and residual service life 

forecasting has several elements, which together provide an 

accurate picture of the machine as a whole. First there is the 

actual inspection and testing. These activities are followed by 

the interpretation of measured data and analysis of the existing 

situation. Interpretation and analysis yield an overview of the 

condition of the machine and its likely residual service life. 

Finally, there is failure analysis. In the event of unexpected 

problems, destructive testing of the failed component can be 

undertaken to ascertain the cause. As well as providing you 

with insight into the condition and residual service life of your 

machinery, we can advise you regarding the most cost-effective 

repair, maintenance and operating strategy. We can indicate 

whether it is best to take a machine out of service straight away, 

to make provision for replacement, or to continue running the 

machine normally. The diagnostic techniques that we use are 

selected to minimize the need to dismantle rotating machinery. 

Dismantling is a costly and time-consuming business, which 

introduces risk; as such it should be avoided except where 

strictly necessary. 

DNV KEMA uses four primary techniques for condition 

assessment and residual service life forecasting: 

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Endoscopy: a visual inspection technique that enables the 

examination of areas that cannot be viewed with the naked eye 

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Dielectric testing: a technique for assessing the electrical 

insulation of rotating machinery components


ENERGY 05 

■■ 

Mechanical engineering inspections: non-destructive testing 

that yields information about the mechanical condition of 

a machine 

■■ 

Failure analysis: analysis with a view to identifying and 

describing the causes and mechanisms of failure 

Decommissioning 

When a rotating machine reaches the end of its economic 

and technical service life, we can advise you about its 

decommissioning. If your installation no longer fullfils the 

applicable requirements, it may well require decontamination. 

This involves the disposal of harmful substances, such as 

asbestos, oil and heavy metals. The decontamination of 

hazardous materials is specialist work. DNV KEMA has the 

expertise and the professional personnel required to oversee 

decontamination activities and verify compliance with the 

applicable regulations. 

DNV KEMA is not associated with any supplier. Moreover, DNV 

KEMA has decades of experience in the diagnostic analysis 

and condition assessment of rotating machinery. We also have 

our own certified laboratories. DNV KEMA is a reliable and 

independent provider of technical and operational services, as 

well as inspection, testing and certification services. We have 

the expertise, facilities, technology and experience to answer 

all your questions about the condition and residual service life 

of your installation. If you wish, we can also provide you with 

decision-making support and advise you on the implementation 

of appropriate measures. 

DNV KEMA offers the following services 

■■ 

Advice 

■■ 

Inspection 

■■ 

Quality control 

■■ 

Diagnostics 

■■ 

Failure analysis 

■ ■ (non-)destructive testing

06 

ENERGY 


DNV KEMA, your partner in 

rotating machinery 

Valuable knowledge with practical applications 

Systems are becoming increasingly complex. Generators are large and applied materials and techniques sophisticated. 

Electricity is generated in a generator, thus being the component and source of all pleasant things in life, hence, the 

source of today’s prosperity. So no one wants to think of the situation when the generator fails to produce electricity. 

General 

The generator is one of the most crucial parts of a nuclear, fossil 

or industrial plant. It is therefore of major importance that 

the generator is always in a perfect condition. They have been 

designed to operate for years without any problems. However, 

when time passes by, condition deteriorates and the chance of 

failure increases and forced outages are not always avoidable. 

DNV KEMA offers a broad range of cutting-edge techniques 

which can be performed within the shortest possible timeframe 

against minimum costs. 

Our services include 

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Retaining ring inspections 

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Bump testing 

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Honing and boresonics examinations 

The results provide insight in the most cost-effective and 

accountable approach for operation and maintenance of the 

generator, now and in the future. 

Independent and reliable failure services 

It never rains, but it pours. This well-known saying is true for 

failures in complex industrial installations, which is why it is vital 

to find the answers to three important questions: what caused the 

failure, might it happen again, and how can this be prevented 

These questions lead to other more detailed questions like: has 

there been an accident, is it a structural fault, or is it due to 

negligence during the installation’s daily operation 

Once failure has occurred, it is already too late. The remaining 

possibility is to evaluate. Many problems have their cause in the 

design phase.


ENERGY 07 

DNV KEMA begins with the analysis of both the failure and the 

generator itself, its mechanical and electrical properties. Apart 

from failure analysis in the laboratory, investigations take place 

on site, which might involve visual examinations, electrical or 

non-destructive testing. To prevent failure in the future, we might 

recommend changes in design, operation and maintenance. In 

the course of their investigations, our experts make use of every 

available tool and technique, including inspections, diagnosis, 

database references and procedural and ethical assessment. We 

have our own world-renowned test laboratories at our disposal, 

equipped with the most advanced equipment. 

Visual inspections and quality assurance 

Problems can be prevented by means of effective quality 

control. It is advisable to keep a close check on quality during 

manufacture, repairs or construction work. DNV KEMA has over 

decades of experience in the technical inspection of electricity 

generating units, transmission systems and substations. Over the 

years, the DNV KEMA inspectors have gained extensive know-how 

concerning potential bottlenecks in the design, production and 

repair processes. It is possible to anticipate such bottlenecks by 

means of an adequate inspection program. 

Inspections can be made during the manufacture of new 

components, during maintenance stops or failures at operating 

installations. 

Besides registering defects, our inspector will suggest solutions, 

giving you access to all of DNV KEMA’s testing and calculation 

facilities and any other expertise you might require, for example 

the 

insulating quality and our knowledge of materials, composites, 

plastics and paint. We can carry out non-destructive tests, failure 

analysis, fitness for purpose analysis and remaining lifetime 

assessments. 

In order to maintain the knowledge and expertise which we have 

gained over the years, a team of DNV KEMA’s experts are united 

in a so-called knowledge platform called CentRoM, which is the 

central organization for rotating machinery. 

We usually use a well-considered approved action plan, but our 

inspectors will base their recommendations on your approach and 

plans, giving you an individual custom-made service. 

Your benefits 

■■ 

By preventing failures, no loss of production & no repair bills 

■■ 

Database with info gathered over 40 years of experience in failure 

analysis 

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Independent organization with objective test results recognized 

worldwide 

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Clear insight in capabilities & limitations of materials & products 

■■ 

Highly qualified staff ensures valuable support in identifying & solving 

your technical problem

08 ENERGY Non-destructive testing & plant diagnostics 

Rotor dynamics and 

electromagnetic fields 

A powerful but dangerous combination 

A rotor is exposed to a complex spectrum of forces of both 

mechanical origin (rotation) and electrical origin (electromagnetic 

fields). Furthermore, a rotor is an assembly of numerous 

components that have to operate in perfect balance and fully 

insulated from one another. The main components of a turbo 

generator rotor are: 

■■ 

Rotor body 

■■ 

Windings 

■■ 

End winding retaining rings 

■■ 

Rotating rectifier 

at very high speed (typically 3,000/3,600 rpm). Furthermore, 

although the rotor experiences great mechanical stress and high 

temperatures (in some cases up to 300°F/150°C) while 

subjected to electrical voltage and current, it is expected to 

function in this manner for years without failure. 

Each component has its own particular properties and therefore 

requires specialist testing and analysis. 

The generator rotor 

The rotor can be visualized as a large rotating electromagnet 

with north and south poles. The generator rotor represents 

an excellent combination of electrical, mechanical and 

manufacturing skills in which the field coils are well insulated, 

supported and ventilated in a compound structure rotating


ENERGY 09 

The three design constraints limiting the size and life of generator 

rotors are temperature, mechanical force and electrical insulation. 

Degradation of the generator field can be caused by a number 

of factors, including a breakdown in insulation due to time and 

temperature and mechanical wear. To understand the intricacies 

of the field winding design, we must remember that the basic 

function of the rotor is to produce a magnetic field of the size and 

shape necessary to induce the desired output voltage in the stator. 

The winding and components should be designed to require little 

maintenance during the 30 or more years of expected operation, 

which is the typical lifetime for a base-loaded power station. 

Rewinds may be more frequent under extreme conditions such as 

an open ventilated gas turbine generator in a dirty environment, 

or frequent start/stops or load cycling. As a generator rotor ages, 

its insulation can be affected by temperature, mechanical 

wear and operating incidents. Rotor forging and other rotor 

components are also at risk. The most common problems 

occurring with generator rotors are shorted winding turns and 

breakdown in groundwall insulation. 

Rotating diode bridge assembly 

There are two types of excitation systems which can be employed 

for generators, the brushless excitation system and the static 

excitation system. With the brushless excitation system the exciter 

consists of two basic component assemblies, the exciter stationary 

field assembly and the exciter rotor comprising the rotating 

exciter armature and a rotating rectifier bridge assembly. 

A turbo generator rotor mainly 

consists of the following components 

■■ 

Rotor body 

■■ 

Windings 

■■ 

End winding retaining rings 

■■ 

Rotating rectifier

10 ENERGY Non-destructive testing & plant diagnostics 

The poles of the stationary field assembly are excited by an 

auxiliary winding in the generator stator via a voltage regulator. 

Optionally, the supply can also be provided by a pilot exciter. The 

three-phase current generated in the rotating armature of the 

main exciter is rectified via rotating diodes and conducted to the 

rotor winding of the turbogenerator via copper conductors in the 

shaft center. Basically, the diodes convert the alternating current 

output of the exciter generator to direct current, which is then 

fed to the rotor winding of the generator. 

DNV KEMA performs visual inspections with regard to the 

functioning of these rotating diodes. With a static excitation 

system the rotor field current to the rotating field winding of 

the generator is supplied via slippings. 

The conductor bundle consists of a number of single conductors 

ranging from a few numbers in very small motors to more than 

200 in large turbine type generators. Depending on the type 

of machine and rated voltage and current the conductors are 

connected in series or in parallel. These conductors have to be 

insulated due to the different voltages. This insulation is stressed, 

but mainly by temperature and voltage transients. A challenge 

on its own is the part of the winding that is outside the core: the 

end windings. The sole function of these windings is to guide the 

current from the conductors in one slot to those in another slot. 

Stators 

The stator of a 3-phase rotating electrical machine, regardless 

whether it is synchronous or asynchronous, is the most expensive 

part of the machine. Here the energy of the magnetic field is 

transferred in electrical energy or vice versa. Phyiscally, the largest 

part of the stator is the core that carries the magnetic flux and 

also holds the windings. Within these windings the conductor 

bundles carry the electric current that flows due to the voltage 

induced by the magnetic flux. The main insulation in between 

the conductor bundle and the core has to deal with dielectric 

stress, temperatrue, different thermal expansion of the core and 

the copper in the conductors and vibrations, and in some cases in 

difficult ambient conditions.


ENERGY 11 

Being built up entirely from copper and insulation materials, 

end windings are complex mechanical constructions that are 

especially vulnerable to transient over currents and suffer easily 

due to vibrations. 

Last, but not least, there is insulation material in the core 

itself which is necessary to avoid circulating currents between 

the core sheets. This material is also stressed, but mainly due 

to temperature. 

Important questions are the quality during start-up, the 

speed of aging, the condition at a given moment in time and 

the remaining lifetime from that moment on. A number of 

(diagnostic) tools has been developed to furnish answers to all 

these questions. 

Generator coolers 

The cooling medium at larger generators is hydrogen (H2). The 

advantage of this gas is its lack of viscosity and sufficient specific 

heat capacity. However, an obvious disadvantage is its inflammable 

character. Leaks in these coolers that are built in the generator 

housing lead to sudden outages and costly repairs. These coolers 

should be checked regularly in order to reduce chances of 

leaking. With Eddy Current technology and combined visual 

inspection this can be performed quite efficiently. There is no 

budgetary reason not to check these during generator revisions, 

whereas the benefits of having early warnings can be large.

12 

ENERGY 


Mechanical testing of rotors 

and stators 

Safe operations with flaw indications 

Retaining ring inspections 

Replacing retaining rings is quite an expensive operation, it may 

even cost up to EUR 1,000,000. The DNV KEMA Inspection 

system of Retaining Rings (KIRR) offers a perfect solution 

to these problems. This system detects and characterizes flaw 

indications in the retaining ring and will enable you to determine 

whether the retaining ring needs to be replaced. Furthermore, 

you will be able to continue monitoring newly detected flaw 

indications that are still acceptable to be worked with. This 

means that you can regularly keep an eye on the condition of the 

retaining rings. Initially DNV KEMA will try to obtain as much 

information as possible about the ring’s geometry and the actual 

access to the rotor, sometimes there is hardly any information 

available. The ring’s geometry is then first measured using an 

ultrasonic C-scan mapping technique. Ultrasonic crack detection 

is focused on the shrunk connections and wall thickness steps. 

The ring’s coating is left in place. Geometry and flaw data 

are then fed into a computer model. The residual life span is 

embedded in the analysis. 

Bump testing of stator end windings 

With bump testing the natural response of an end winding is 

measured upon excitation with a mechanical impulse (hammer). 

The response is fully determined by the properties and 

mechanical structure of the stator end winding. This principle is 

comparable to the excitation of piano strings by their hammers 

when vibrating in distinct frequencies and thus producing a tone. 

A relative simple method when compared with the end winding, 

but nevertheless the natural vibration frequencies of a complex 

part can be measured accurately. An acceleration transducer 

is mounted successively on each end winding unter test. The 

measured signals are fed into a data analyzer and the response 

frequencies and magnitudes, the so-called Power Spectrum 

Density (PSD), is recorded, showing the energy distribution of 

vibrations. An extra acceleration transducer is mounted at the 

hammer. The most important frequency range for analysis lies 

between 95 and 110 Hz. Depending on temperature resonance, 

modes can shift slightly during operation. Electromagnetic 

forces at the windings have a frequency of 100 Hz. When natural 

vibrations lie close to 100 Hz, resonance may occur resulting in


ENERGY 13 

high movement of the end windings and high wear, resulting 

in even higher vibration magnitudes et cetera, culminating 

in collapse. The frequency spectrum which is measured by its 

acceleration transducer lies in the range of 20 to 200 Hz and 

delivers the “base spectrum” that is induced in the object under 

test. Frequency Response Analysis is used with which the obtained 

frequency spectra from the end winding are divided with this base 

spectrum, yielding a “normalized” signal. 

Wedges 

In order to support windings of electrical machines in the slots 

of rotors or stators, various wedges are used that are capable 

of withstanding the mechanical and electrical stresses acting 

on the windings during steady state and dynamic behavior of 

the machine. In combination with liners, blocks and other 

materials, they maintain the position of slot windings and end 

windings, without causing heavy stray currents, short-circuit 

currents, and without vibration of the windings. Furthermore, 

all these tools are capable of withstanding the high temperature 

stresses in the machine. It is of paramount importance that 

the wedges and other supporting materials do not loose their 

supporting capabilities during service, that these are not subject 

to degradation and that they remain fixed, without any space for 

vibration, noise generation, risk of getting loose and that under 

no circumstances any part may interfere with the rotation of 

the machinery. Any refurbishing of the machine shall include 

a thorough inspection and check of all winding supporting 

materials. 

Generator bore examinations “BoreSonics” 

Boresonics encompasses a series of operations and examinations 

conducted on generator rotors with central bores through 

their forgings. Lessons are learned from a catastrophic event, 

as such is the case for the in-service practice of examining the 

near surface volume of material in generators with central 

bores. The honing process serves to remove any oxides that 

may have formed that would interfere with subsequent surface 

examinations such as visual, magnetic particle or Eddy currents. 

It also provides a means to remove a layer of material that may 

have become embrittled and subject to crack initiation. The 

process also produces a smooth uniform surface that resists stress 

concentrations. 

DNV KEMA has developed a field transportable horizontal honing 

system. This system provides surface preparation on bores from 70 

to 250 mm (2.75 – 10 inches) in diameter, and bore lengths of up 

to 13.2 meters (40 feet). The system allows you to let DNV KEMA 

accomplish a full range of tasks associated with the required bore 

inspections, like: 

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Visual examination 

■■ 

Magnetic particle examination 

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Eddy current examination 

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Ultrasonic examination 

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Follow-up services

14 

ENERGY 


Electric testing of rotors 

DC-resistance of the rotor windings 

In order to detect the presence of defects in the rotor windings 

current path, the DC-resistance of the stator windings is determined 

by means of a DC-current, which is conducted through the rotor 

winding in series with a calibrated shunt. The test is normally 

performed between the slip rings. The DC-current must be large 

enough to enable accurate measurements, but shall not result in 

significant thermal effects during the measurement. The voltage 

across the shunt and the rotor winding is compared. During the 

measurements the temperature of the winding is recorded. 

For evaluation of the results a comparison is made with previous 

measuring results, such as obtained by the manufacturer in the 

factory. 

Voltage withstand tests 

on the rotor windings 

Verification of the condition of the electrical coil insulation of rotor 

windings may be done by Megger testing followed by application of a 

prescribed power frequency voltage for some time. The coil insulation 

to earth or to neighbouring coils is tested with short-circuited rotor 

windings. Generally the voltage will be less than the level of the original 

factory acceptance test. 

For this purpose the results mostly are corrected to a temperature 

of 75°C. If the results are deviating from the expected values 

the cause should be traced. Any bad connection or defect in the 

terminals or (partly) interruption in the windings or leads may 

cause large damage to the machine. 

Impedance of the rotor windings 

In order to detect presence of defects in the rotor windings 

current path, the 50/60 Hz power frequency impedance of the 

stator windings is determined by means of an AC-current, which 

is conducted through the rotor winding. Then voltage across the 

slip rings is measured. The test is normally performed between 

the slip rings. The AC-current must be large enough to enable 

accurate measurements but shall not result in significant thermal 

effects during the measurement. Subsequently the 50/60 Hz 

power frequency impedance of the rotor winding was determined. 

The accuracy of the measurement depends on several factors. 

For instance the wave shape of the harmonic current may be 

crucial. For evaluation of the results, a comparison is made with 

previous measuring results, such as obtained by the manufacturer 

in the factory.


ENERGY 15 

RSO-measurement on the rotor windings 

In order to test the performance of the rotor windings RSO 

(Recurrent Search Oscillogram) measurements are conducted. 

For the test a transient signal with respect to the rotor shaft is 

injected in each of the slip rings. A voltage impulse generator is 

connected between one of the slip rings in turn and the rotor 

shaft. The impulse voltage and the resulting current are fed into a 

transient recorder. The admittance as a function of the frequency 

will be determined as the twenty-fold logarithmic ratio of current 

and voltage, for each frequency component resulting from the 

Fourier transform and is expressed in dB. From the resulting 

current the admittance can be derived. The admittance of the 

two branches of the rotor winding relative to the rotor shaft will 

be measured. The admittance characteristics should show no 

difference. 

The test is performed with the rotor in standstill mode, or 

if possible, in running mode, at various speed adjustments. 

Additional information about the condition of the rotor windings 

can be found by comparison of the transient behavior of 

symmetrical winding branches, each connected to a slip ring. 

Insulation resistance of the rotor windings and 

determination of the polarization index 

To determine the polarization index and insulation resistance of 

rotor windings a dedicated test can be performed. The insulation 

resistance of the rotor windings is measured by means of a digital 

Megger. During the measurement a DC-voltage of approximately 

500 V is applied. The measurements are performed as a function 

of the time after application of the voltage. The measurements 

are performed on the rotor winding with the other machine 

winding earthed, together with the frame of the stator. 

It is important to register the temperature of the winding for the 

evaluation of the result. 

The leakage current that occurs when the winding insulation is 

subjected to a DC-voltage is a measure for the condition of its 

insulation. A distinction can be made here between the timedependent 

effects during the charging of the winding capacitance 

and the non-time-dependent effects which occur as a result of 

moisture and/or tracking inside or on the surface of the winding 

insulation. The leakage current is measured at a specified DCvoltage 

level by means of an insulation resistance meter (Megger). 

From the measured insulation resistance as a function of the time, 

the polarization index is determined. The ratio of the insulation 

resistance values measured 10 minutes and 1 minute after the 

application of the DC-voltage, is called the polarization index. The 

polarization index and the insulation time constant are measures 

for the evaluation of the insulation condition with respect to 

cleanliness and the presence of moisture. The temperature of the 

winding is of importance. 

The admittance is presented in curves for the circuit inner slip ring to 

shaft (curves inner long and inner short) and for the circuit outer slip 

ring to shaft (curves outer long and outer short). The expression short 

and long relates to the length of the applied interconnecting cable wires 

between pulse generator and rotor. The length of the cable wires is of 

minor importance on the measurement. Furthermore, for frequencies up 

to 20 kHz, the admittance of both circuits is identical.

16 

ENERGY Non-destructive testing & plant diagnostics 

Electric testing of stators 

Reliable operation of a generator is highly dependent on the electrical and mechanical integrity of stator winding. Operating 

temperature, operating voltage stress and thermal-cycle stress cause damage to the stator insulation. 

Failure of a generator stator conductor due to electrical or 

mechanical problems will lead to long forced outage of the unit 

resulting into huge revenue loss. Therefore it is necessary 

for judgment of insulation deterioration to carry out a 

diagnostic test of the stator coil. 

To evaluate the residual life from experience and the 

accumulated data, DNV KEMA offers a full set of diagnostics to 

assess the generator stator condition, like: 

■■ 

Insulation resistance 

■■ 

Polarization index 

■■ 

Capacitance and dielectric losses 

■■ 

Partial discharge 

■■ 

Voltage tests 

Measurement of the DC-resistance of the 

stator windings 

In order to detect presence of defects in the stator windings 

current path, the DC-resistance of the rotor windings is 

determined by means of a DC-current, which is conducted 

through the stator winding in series with a calibrated shunt. 

The test is normally performed between the slip rings. 

The DC-current must be large enough to enable accurate 

measurements but shall not result in significant thermal effects 

during the measurement. The voltage across the shunt and 

the stator winding is compared. During the measurements the 

temperature of the winding is recorded. 

For evaluation of the results a comparison is made with previous 

measuring results, such as obtained by the manufacturer in 

the factory. For this purpose the results are mostly corrected 

to a temperature of 75°C. If the results are deviating from the 

expected values the cause should be traced. Any bad connection 

or defect in the terminals or (partly) interruption in the 

windings or leads may cause large damage to the machine. 

Insulation resistance and polarization index of 

stator windings 

To determine the polarization index and insulation resistance 

of stator windings a dedicated test is performed. The insulation 

resistance of the stator windings is measured by means of a 

digital Megger. During the measurement a DC-voltage is applied 

ranging from 500 – 5000 V. The measurements are performed 

as a function of the time after application of the voltage. The 

measurements are performed on the stator winding with the


ENERGY 17 

other machine winding earthed, together with the frame of the 

rotor. It is important to register the temperature of the winding 

for the evaluation of the result. 

A distinction can be made here between the time-dependent 

effects during the charging of the winding capacitance and 

the non-time-dependent effects which occur as a result of 

moisture and/or tracking inside or on the surface of the 

winding insulation. From the measured insulation resistance as a 

function of the time, the polarization index is determined. The 

ratio of the insulation resistance values measured 10 minutes 

and 1 minute after the application of the DC-voltage, is called 

the polarization index. The polarization index and the 

insulation time constant are measures for the evaluation of the 

insulation condition with respect to cleanliness and the presence 

of moisture. The temperature of the winding is of importance. 

For modern epoxy-resin insulated windings the insulation 

resistance should be more than 1000 MΩ at 20˚C and the 

polarization index PI should be in accordance with the IEEE 43 

standard. 

Capacitance and dielectric losses (Tan-δ) 

Dielectric losses and capacitance of the winding will increase 

with the voltage because voids (gaps) inside the insulation will 

be short-circuited as a result of occurring partial discharges. 

The dielectric losses of the insulation represent the relation of 

the resistance to the capacitive impedance. The dielectric losses 

depend on temperature and frequency. The capacitance of the 

winding at 0.2U n is of vital importance to find out whether major 

changes have occurred in the insulation. The capacitance of 

the windings depends on the relative dielectric constant of the 

insulation. The capacitance is subject of voids and gaps in the 

insulation. 

For the purpose of these measurements a Schering bridge is 

used with a loss-free standard capacitor. The measurements are 

performed as a function of the voltage between for example 0.2 

and 1.0U n in steps of 0.1U n. 

The measurements can be performed part-wise, mostly per 

phase, on the individual parts of winding with the other parts 

connected to earth and to the stator frame. The dielectric 

loss angle of the insulation and the capacitance of the stator 

windings are determined. The measurement can be done very 

accurately and reliable. 

In order to assess the condition of 

the generator stator, DNV KEMA offers 

a full set of diagnostics such as 

■■ 

Insulation resistance 

■■ 

Polarization index 

■■ 

Capacitance and dielectric losses 

■■ 

Partial discharge 

■■ 

Voltage tests

18 

ENERGY 


From the average value of the maximum rise of Tan-δ with 

voltage, compared with the values obtained during previous 

measurements, it can be determined whether the volume of 

air inside the insulation has changed. The relative capacitance 

change is a measure for the total air volume which is shortcircuited 

by internal partial discharges. 

Partial discharges 

Partial discharges may occur due to discontinuities in the 

insulation. Partial discharges give an indication of abnormalities. 

It is important to know how the test is performed. The partial 

discharge measurements are performed with a multi-channel, 

phase-related partial discharge detector. Prior to each test, a 

calibration is performed on each side of the generator winding 

by means of an impulse-generator giving a repeating pulse with 

known partial discharge intensity. During the partial discharge 

measurements the entire winding - from phase connection to 

star point - is energized at a certain voltage level with respect to 

earth. In practice, when the machine is under normal service 

conditions, the voltage is distributed equally along the winding 

from the star point of the machine to the phase connection. 

Different partial discharges: 

■■ 

Slot discharges occur in a slot in the stator between the bar 

including its insulation and the stator. This can be the result 

of degradation or damage to the semi-conducting layer on the 

surface of the bar insulation. 

The partial discharges are recorded during a period of approximately one 

minute at voltage levels of 0.6U n and 1.0U n at a frequency of 50 Hz. Based 

on these measurements, graphs can be plotted in which the intensity of 

the partial discharge level is presented as a function of the phase angle at 

which the discharge occurred. Within these graphs, different intensities are 

presented in different colors. The graphs can contribute to the interpretation 

of the test result, since the shape of a partial discharge pattern depends 

on the location where the discharges occur. Some of the recorded patterns 

show an asymmetry between the discharges in the positive and negative 

half-cycle of the voltage sine wave. This is typical for discharges other than 

internal discharges, which show a more symmetrical pattern.

Non-destructive testing & plant diagnostics ENERGY 19 

■■ 

End winding discharges can occur at the location where the bar 

extends out of the slot. To prevent partial discharges a corona 

protection layer is applied at this location. When this layer is 

damaged, partial discharges can occur here. 

■■ 

Internal discharges occur in the insulation around the 

conductors, mostly caused by detachment of the different 

layers in the insulation (delamination). Gaps/voids, originated 

during the manufacturing process, can also be the cause for 

this type of partial discharges. 

■■ 

Bar-to-bar discharges can occur at the end winding between 

two bars of different phases or between a bar at high-voltage 

and a neutral bar. In general, this type of discharge can occur 

between two separate bars next to each other when a too high 

voltage difference exists between them resulting in a too high 

electrical field. 

Voltage withstand tests on the stator windings 

As a further check on the insulation of the stator windings, and 

depending partly on the size of the generator (or motor), it 

may be desirable to perform a voltage test, as well as measuring 

the capacity, loss angle and partial discharges. The test method, 

appropriate voltage strength and test duration are preferably 

decided on the basis of consultation with the user and 

manufacturer. 

ElCID test 

Defects in inter-laminar insulations of stator cores in rotating 

electrical machines such as generators and electro motors cause 

fault currents to flow locally in the core, which may result in 

local overheating or hot spots in damaged areas. These hot 

spots should be detected and repaired during routine machine 

overhauls. The conventional testing method for measuring these 

core faults is known as the full ring flux testing method. This 

methodology requires the core to be excited to near its normal 

working flux level for a period of time. 

An alternative method to detect faults in core inter-lamination 

insulation by electromagnetic means was developed in 1978. 

Instead of the full flux working level, this newer method uses 

only a small fraction of excitation to generate fault currents 

within the core body which are sensed by a pick-up coil. This 

provides an accurate indication of damaged parts, including 

tooth tips and slot walls, and possible surface damage, without 

the normal problems when testing with high excitation. This 

methodology is called the ELectromagnetic Core Imperfection 

Detector (ELCID) and is accepted worldwide for reliable and 

safe detection of stator core inter-laminar faults. 

ELCID operates at only 4% of the normal operating flux. This 

low flux is generated by a portable, quickly installed excitation 

kit. Any imperfection in the core inter-laminar insulation 

produces fault currents, which are detected by a Chattock 

coil and analyzed by the ELCID signal-processing unit. These 

Chattock coils are mounted on a Generator Inspection Vehicle 

(GIV), which is developed by DNV KEMA, in order to perform 

the ELCID test without necessity of removing the rotor. 

Measurement results are digitally stored for analysis and report 

generation to precisely locate the faults in the stator core. 

Future results can be compared to past results for trend analysis. 

This GIV is also used when testing stator wedges on integrity and 

eventual detachment (tap testing). The vehicle is then provided 

with equipment for mechanical excitation of the wedges and 

detection of the impulse response (WTA: Wedge Tightness 

Assessment). Cameras are mounted on the GIV in order to 

perform various visual inspections. 

Key benefits of the ELCID test 

■■ 

Tests are repeatable 

■■ 

Immediate test results available for local analysis 

■■ 

Determination of exact location of defects: on the (sub)surface 

or under conductors 

■■ 

Tests with or without windings in place 

■■ 

Allows partial retests of a core, results can be merged to obtain 

a complete view of the core condition 

■■ 

Tests can be performed with the rotor in-situ

N.V. KEMA 

Utrechtseweg 310, 6812 AR Arnhem, The Netherlands I Tel: +31 26 356 9111 I Fax: +31 26 443 4025 

emea@dnvkema.com I www.dnvkema.com 

About DNV KEMA Energy & Sustainability 

DNV KEMA Energy & Sustainability, with more than 2,300 experts in over 30 countries around the world, 

is committed to driving the global transition toward a safe, reliable, efficient, and clean energy future. 

With a heritage of nearly 150 years, we specialize in providing world-class, innovative solutions in the fields 

of business & technical consultancy, testing, inspections & certification, risk management, and verification. 

As an objective and impartial knowledge-based company, we advise and support organizations along the 

energy value chain: producers, suppliers & end-users of energy, equipment manufacturers, as well as 

government bodies, corporations and non-governmental organizations. DNV KEMA Energy & Sustainability 

is part of DNV, a global provider of services for managing risk with more than 10,000 employees in over 

100 countries. 

Present around the globe 

■■ 

Management & Operations Consulting 

■■ 

Cleaner Energy Services 

■■ 

Electricity Transmission & Distribution 

■■ 

Sustainable Use Services 

■■ 

Transportation Systems 

■■ 

Gas Consulting & Services 

■■ 

Testing, Inspections & Certification 

■■ 

Accredited Climate Change Services 

Office locations 

■■ 

Office locations 

Agents / Business partners 

Agents / Business partners 

ETD.12012.E&O.05

Diagnostics and testing of rotating electrical machines - DNV Kema

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